Apparatus for reducing heterostructure acoustic charge transport device saw drive power requirements

ABSTRACT

A HACT device employing a thin-film overlay of a more strongly piezoelectric material can operate as a delay line and as a tapped delay line, or transversal filter, while requiring less total power for the SAW clock signal. The increased electrical potential per unit total SAW power thus realized facilitates coupling between the total SAW energy and the mobile charge carriers. Some materials systems, such as a GaAs substrate and a ZnO thin-film overlay, will require an intervening thin-film dielectric layer in between the HACT substrate and epitaxial layers and the thin-film piezoelectric overlay. This may be necessitated by chemical, semiconductor device processing, or adhesion incompatibilities between the substrate material and the thin-film overlay material.

This application is a continuation-in-part of prior application Ser. No.07/836,165, filed Feb. 18, 1992, now abandoned, which is a continuationof prior application Ser. No. 577,180, filed Sep. 4, 1990, nowabandoned.

BACKGROUND OF THE INVENTION

The present invention pertains to a heterostructure acoustic chargetransport (HACT) devices and more particularly to an arrangement forimproving device performance by reducing the magnitude of the surfaceacoustic wave (SAW) required for effective HACT device operation.

An HACT device employs a powerful ultra high frequency (UHF) SAWpropagating on the top, highly polished surface of a wafer ofpiezoelectric semiconductor material, usually gallium arsenide (GaAs),to bunch mobile charge carriers in extrema of the SAW electricalpotential and to then transport these discrete chart packets at thespeed of sound through semiconductor material, as is described in detailin U.S. Pat. No. 4,893,161, entitled "Quantum-Well Acoustic ChargeTransport Device," issued to William J. Tanski, Sears W. Merritt andRobert N. Sacks. Further information regarding ACT and HACT devices isfound in U.S. Pat. No. 4,980,596, entitled "Acoustic Charge TransportDevice Having Direct Optical Input", issued to R. N. Sacks et al.; U.S.Pat. No. 4,926,083, entitled Optically Modulated Acoustic ChargeTransport Device", issued to S. W. Merritt et al.; U.S. Pat. No.4,884,001, entitled "Monolithic Electro-Acoustic Device Having AnAcoustic Charge Transport Device Integrated With A Transistor", issuedto R. N. Sacks et al.; and U.S. Pat. No. 4,633,285, entitled "AcousticCharge Transport Device And Method" issued to B. J. Hunsinger et al.;the above-noted patents are incorporated herein by reference. The SAWthus functions as an acoustic clocking signal, similarly to clockingsignals in a conventional charge-coupled device (CCD), but without needfor complex interconnections which CCDs require. SAW transducer/channelwidth design considerations are addressed in "A Synopsis of SurfaceAcoustic Wave Propagation on {100}-Cut <110>-Propagating GalliumArsenide" by W. D. Hunt et al., (J. Appl. Phys. 69(4), pp. 1936-1941,Feb. 15, 1991), which is incorporated herein by reference.

CCDs, bucket brigade devices and related memory devices have thedisadvantages that great fabrication complexity is necessary in order toprovide the polyphase clocking signals required by such devices. Otherdisadvantages of such devices include limited practical clocking speedand substantial dark currents. The capacitive nature of the transferelectrodes to which the clocking signals are applied exacerbatesdifficulties involved in attempting to increase clocking speeds becausethese types of charge transfer devices tend to operate best when drivenby clocking signals having sharp transitions, e.g., square waves, whichare difficult to supply to capacitive loads such as clocking electrodes,especially at high frequencies.

Another class of device which has been experimentally demonstrated atlow clocking frequencies (e.g., less than 100 MHz) but which has notshown transfer efficiencies of practical value even at these lowclocking frequencies are based on metal-insulator-semiconductor (MIS)technology wherein a layer of strongly piezoelectric material is appliedover a layer of native oxide to a semiconductor material such assilicon. These devices operate by first establishing a channel region atthe semiconductor-oxide interface by means of an electrical bias whichgives rise to an inversion layer (e.g., a two-dimensional minoritycarrier sheet) which is then subsequently bunched and synchronouslytransported by a SAW (or other acoustic wave) launched from a SAWtransducer which is in proximity to the piezoelectric layer. Suchdevices have consistently had serious problems in practice includinghigh surface state densities at the semiconductor-oxide interface,giving rise to charge trapping effects and poor charge transferefficiency even at low clocking frequencies, high acoustic propagationlosses (limiting practical channel lengths and hence storage times),incompatibility with Schottky electrode sensing techniques fornon-destructive charge sensing, substantial dark currents due to thehigh electrical fields necessary to provide inversion layers and poorinput-output isolation due to the necessary presence of a conductivelayer immediately beneath the charge-carrying inversion layer (inversionlayers can only be formed by biasing a conductive layer). A particularproblem has been that most dielectric films provide high acousticpropagation losses. For these and other reasons related to deviceprocessing problems, acoustically clocked MIS structures have neverproven practical as charge transfer devices. Examples of concepts forsuch devices are briefly mentioned in passing in Boyle et al., U.S. Pat.No. 3,858,232, issued Dec. 31, 1974 and in more detail in Mikoshiba etal., U.S. Pat. No. 4,799,244, issued Jan. 17, 1989. Experimentalperformance of such devices is discussed in Tsubouchi et al., "ChargeTransfer by Surface Acoustic Waves on Monolithic MIS Structure", 1978IEEE Ultrasonics Symposium Proceedings, pp. 20 through 24 whiletheoretical predictions of performance absent charge trapping arepresented in "Modelling of Charge Transfer by Surface Acoustic Waves ina Monolithic Metal/ZnO/SiO2/Si System" by F. Augustine et al., IEEETransactions on Electron Devices, ED-29, No. 12, Dec. 1982, pp. 1876through 1883. In "Fabrication-Related Effects in Metal-ZnO-SiO2-SiStructures", Cornell et al., Applied Physics Letters, Vol. 31, No. 9,Nov. 1, 1977, pp. 560 through 562, provides conclusive experimentalevidence that the sputtering process used to deposit ZnO causes surfacestate problems which severely limit the performance of acousticallyclocked devices using such structures. The above-noted patents andarticles are hereby incorporated by reference.

The very weak piezoelectricity of GaAs (k² =7.4×10⁻⁴ for the Rayleighmode on {100}-cut, <110>-propagating GaAs) dictates that the greatmajority of energy in the SAW is manifested as mechanical energy andonly a small portion of the total energy is manifested through theelectrical potential which accompanies the SAW. It is this electricalcomponent of total SAW energy which bunches charge carriers formingdistinct packets and transporting these packets, representing the inputsignal, through the HACT device. Accordingly, present day ACT and HACTdevices require large (about one Watt) acoustic power levels in order torealize the voltage required (about one Volt) to effect coherent chargepacket transport within the HACT channel, synchronous with the SAW clocksignal.

What is needed is a device architecture providing high charge transferefficiency, long charge storage times, good frequency response, lowclocking signal power requirements and low dark current.

Therefore, it is an advantage of the present invention to provide anHACT device which includes a greatly reduced acoustic power requirementfor achieving coherent, synchronous charge transport.

SUMMARY OF THE INVENTION

In accomplishing the advantages of the present invention, a novel HACTdevice structure employing a thin-film overlay of another material isshown.

A heterostructure acoustic charge transport device includes asemiconductor substrate which has a surface and a source of electricalcharge. The semiconductor substrate also includes a surface acousticwave device which is coupled to the semiconductor substrate. The surfaceacoustic wave device operates in response to the applied electricalcharge source to transport electric charge.

The semiconductor substrate also includes a channel which is disposedalong the surface of the semiconductor substrate. The channel transportsthe electric charge in a particular direction in response to the surfaceacoustic wave device. Lastly, the semiconductor substrate includes apiezoelectric layer which is disposed over the channel. Thepiezoelectric layer facilitates transportation of the electric charge inthe particular direction.

The above and other features and advantages of the present inventionwill be better understood from the following detailed description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal cross-sectional view of an HACT deviceemploying a thin-film overlay of a dielectric and also another thin-filmlayer of material.

FIG. 2 is a graph of the electromechanical coupling coefficient versusZnO layer thickness on a semi-insulating {100}-cut, <110>-propagatingGaAs substrate.

FIG. 3 is a graph of element factor attenuation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A broad variety of different acoustic wave types have application inmicrowave acoustic devices such as that claimed herein, includingsurface acoustic waves (SAWs), also known as Rayleigh waves; surfaceskimming bulk acoustic waves (SSBWS or SSBAWs); pseudo surface waves orleaky surface waves; shallow bulk acoustic waves (SBAWs); surfacetransverse waves (STWs); Stonely, Sezawa, Love and other plate andhigher order acoustic guided waves; longitudinal and shear bulk acousticwaves (BAWs) and the like. For convenience of explanation, the presentinvention is described in terms of SAWs, with the understanding thatother varieties of acoustic propagation are also applicable, includingbut not limited to those listed above.

The terms "surface acoustic wave", "acoustic wave" and "surface wave" or"SAW" are employed interchangeably herein to stand for any suitable typeof acoustic wave propagation. The terms "substrate material","substrate" and "acoustic wave propagating substrate" are employedinterchangeably herein to stand for any substrate supporting acousticwave propagation.

A HACT device employing a thin-film overlay of a more stronglypiezoelectric material can operate as a delay line and as a tapped delayline, or transversal filter, while requiring less total power for theSAW clock signal. The increased electrical potential per unit total SAWpower thus realized facilitates coupling between the total SAW energyand the mobile charge carriers.

Some materials systems, such as a GaAs substrate and a ZnO thin-filmoverlay, will require an intervening thin-film dielectric layer inbetween the HACT substrate and epitaxial layers and the thin-filmpiezoelectric overlay. This may be necessitated by chemical,semiconductor device processing or adhesion incompatibilities betweenthe substrate material and the thin-film overlay material.

In acoustic charge transport (ACT) and in HACT technology, chargetransport occurs in piezoelectric semiconductors (for example,{100}-cut, <110>-propagating GaAs) when mobile charge is injected into,and trapped within extrema of, the propagating electrical potentialwhich is associated with a SAW. Referring to FIG. 1, semiconductorsubstrate 1 has incorporated upon it an interdigital metallic SAWtransducer pattern 2 for generating the SAW in response to an appliedexternal voltage having the appropriate frequency. SAW transduction isenhanced by means of an appropriate thin-film overlay 3 of apiezoelectric material, such as zinc oxide (ZnO), as discussed in "GaAsMonolithic SAW Devices for Signal Processing and Frequency Control," byT. W. Grudkowski, G. K. Montress, M. Gilden and J. F. Black, IEEECatalogue No. 80CH1602-2, pages 88-97, aluminum nitride (AlN), asdiscussed in "Growth and Properties of Piezoelectric and FerroelectricFilms," by M. H. Francome and S. V. Krishnaswamy, in Journal of VacuumScience and Technology A 8(3), pages 1382-1390, or lithium niobate(LiNbO₃) or other ferroelectric material, as discussed in the above andin "Metalorganic Chemical Vapor Deposition of PbTiO₃ Thin Films," by B.S. Kwak, E. P. Boyd and A. Erbil, in Applied Physics Letters, 53(18),pages 1702-1704, which articles are incorporated herein by reference.The travelling-wave electrical potential associated with the SAW is alsoenhanced by overlay layer 3. The SAW then propagates from left to right,for example, encountering the edge of epitaxial layers comprisingchannel 4, sweeping past input contact 5, where charge is injected intoSAW electrical potential minima and entrained thereby. The amount ofcharge injected into any particular minimum of the travelling-waveelectrical potential associated with the SAW is determined by the inputsignal magnitude of impressed upon input contact 5 when the SAWelectrical potential minimum passes by input contact 5 and thisparticular charge packet magnitude remains invariant as the chargepacket thus formed moves through HACT channel 4 at the speed of sound.

Input contact 5 is typically an ohmic contact to the semiconductormaterial comprising channel 4 and is commonly realized by depositing aeutectic mixture of AuGe (88:12) approximately one to two thousandAngstroms thick, followed by a few hundred Angstroms of Ni, for example,and which is alloyed to the semiconductor material by heating in areducing atmosphere.

Rapid thermal and/or laser annealing, heating in a hydrogen furnace andother techniques well known in the semiconductor arts are successfullyemployed to form such ohmic contacts. Alternatively, other techniquesknown in the relevant arts for forming ohmic contacts may be employed.

The input electrode bias is formed by impressing a voltage differencebetween input ohmic contact 5 and input Schottky gate 6. Charge packetsthe move beneath nondestructive sensing (NDS) Schottky electrode array7A and sensing electrode arrays 7B and 7C where charge capacitivelycouples to NDS electrodes 7A and sense electrodes 7B and 7C and formsimage charges which replicate the channel charge and hence the chargerepresenting the input signal.

NDS Schottky electrodes 7A are formed directly on the semiconductormaterial forming a top surface of channel 4. This is not possible forMIS-type devices wherein an inversion layer is employed becauseinversion layers forward bias Schottky contacts, draining chargecarriers from and collapsing the inversion layer. In the SST devicearchitecture disclosed herein, NDS electrodes 7A may be formed either onthe top surface of channel 4, while additional sense electrodes 7Bbetween dielectric layer 11 and piezoelectric layer 3 or further senseelectrodes 7C may be formed atop piezoelectric layer 3. Thisflexibility, which is not possible for MIS type structures for reasonsnoted hereinabove, allows for greater design freedom than is possiblewith prior art ACT and HACT devices or with MIS type structuresutilizing inversion layers.

Electrodes in HACT and ACT devices may in principle comprise anyconductive material having appropriate acoustic properties (E.g.,aluminum). However, the presence of and the power levels required forthe SAW clocking signal as well as the unique concerns associated witheffecting charge transport impose criteria for electrode composition andperformance distinct from those of SAW filters, for example.

Acoustically clocked charge transport devices require (i) highconductivity electrode materials having (ii) high resistance to electro-and acousto-migration together with (iii) low acoustic propagationlosses and providing electrodes having (iv) minimal acoustic reflectioncoefficients.

Requirement (i) must be met in at least the acoustic wave transducerarea in order to reduce heating and attendant velocity and/or frequencyshifts and also to prolong transducer operating life. Requirements (ii)through (iv) are significant throughout device length and apply to allelectrodes exposed to ultrasound.

Requirement (iv) is met through choice of materials of similar densityand stiffness to the material(s) on which the electrodes are disposed,through reduction of electrode thickness (typically electrodes are lessthan a few thousand Angstroms thick and may be only several hundred to athousand Angstroms in thickness) and by employing electrode arrangementswherein the reflection coefficients of some electrode edges cancel thereflection coefficients of other electrode edges (e.g., use ofelectrodes one-eighth or one-sixth of a clock wavelength wide disposedon one-fourth or one-third of a clock wavelength centers, respectively)or by burying the electrodes in the substrate to minimizediscontinuities (see, for example, U.S. Pat. No. 4,499,440, entitled"Low Reflectivity Electrodes In Semiconductive SAW Devices", by T.Grudkowski, issued Feb. 12, 1985, which is incorporated herein byreference).

Examples of materials satisfying requirements (i) through (iv) areprecipitation-hardened aluminum alloys as described, for example, inU.S. Pat. No. 4,906,885, entitled "Electrode Material for SurfaceAcoustic Wave Devices and Surface Acoustic Wave Device Using The Same"by H. Kojima et al., issued Mar. 6, 1990; in U.S. Pat. No. 4,942,327,entitled "Solid State Electronic Device" by H. Watanabe et al., issuedJul. 17, 1990; and in U.S. Pat. No. 5,039,957, entitled "High PowerSurface Acoustic Wave Devices Having Copper And Titanium Doped AluminumTransducers And Long Term Stability" by J. Greer et al., issued Aug. 13,1991, which patents are incorporated herein by reference. In NDSelectrode arrays, it is important to reduce clock signal effects at theNDS array output while maintaining large-signal interactions between theclocking signal and the transported charge (the electrode array must notshort-circuit the clocking potential).

This latter need may be realized through sparse sampling (placingsensing electrodes coupled to an output at separations greater than aclock signal wavelength) or, for example, by an array of one-eighth of awavelength wide electrodes having one-fourth of a wavelength pitchwherein every fourth electrode may be coupled to a common buss and theclock signal removed by filtering the output signal derived from thecommon buss(es).

Image charges induced in NDS electrodes 7A and/or sense electrodes 7Band/or 7C are combined to form a transversal filter from which an outputsignal is derived.

A fundamental distinction between MIS-type devices and SST-type devicesis apparent in comparison of the element factors (FIG. 3) appropriate toNDS electrodes. As representatively illustrated in FIG. 3, NDS elementfactor attenuation is dramatically less pronounced for NDS electrodesimmediately above the charge packets than is the case for electrodesplaced atop insulator structures, etc.

In FIG. 3, logarithms of element factors (vertical axis) are shown forseveral charge packet depths versus signal frequency divided by theNyquist frequency. The Nyquist frequency is simply the clockingfrequency divided by two and which arises from the well-known Nyquisttheorem.

In FIG. 3, reference level 305 corresponding to no element factor isshown above trace 310. Trace 310 corresponds to a HACT-type structureemploying, for example, Schottky electrodes placed directly upon thechannel surface and illustrating, for example, less than 1 dB of elementfactor attenuation at the Nyquist frequency.

Middle curve 315 illustrates a representative element factorcorresponding, by way of example and not intended to be limiting, to asense electrode placed atop dielectric material (e.g., an oxide ornitride layer such as dielectric layer 11 of FIG. 1, etc.) while bottomtrace 320 corresponds to an element factor appropriate to a senseelectrode placed farther above the channel (e.g., atop piezoelectriclayer 3 of FIG. 1).

An element factor appropriate to the type of charge transfer devicedescribed in Hunsinger et al. (supra) provides approximately 10 dB ofattenuation at the Nyquist frequency, a value appropriate for chargetransport occurring at a depth of about 0.375 of the acoustic clockingsignal wavelength below a sense electrode where the electrode isrelatively narrow (e.g., one-eighth of a wavelength of the acousticclocking signal broad).

The amount of element factor attenuation varies with separation of thecharge packet from the sensing structure (i.e., the farther the chargeis below the electrode, the greater the attenuation).

The well-known element factor, originally developed in the context ofantenna theory, multiples that frequency response predicted by simplytaking the Fourier transform of the weights of the sense electrodes. Asa result, high frequency performance is substantially degraded from thatotherwise possible when the thickness of the material between the senseelectrode and the charge packets is increased.

Charge packets are then swept to delay line output contact 9, where adelayed replica of the input signal is extracted by application of asuitable bias voltage applied between delay line output Schottky gatecontact 8 and delay line output contact 9. Finally, SAW energy isincident upon acoustic absorber 10 consisting of a mass of a suitableviscous material, such as a room temperature vulcanizing silicone rubber(RTV), to avoid unwanted effects resulting from reflection of SAW energyby edges of substrate 1.

In heterostructure acoustic charge transport devices, is is importantthat the electrical potential of the acoustic wave not be shortcircuited by metallic structures at the channel surface. Accordingly,electrodes which are substantially less than an acoustic wavelength inwidth (in the direction of acoustic wave propagation) are preferred.

Reduction of acoustic reflections from electrodes is also a concern. Ifsubstantial acoustic energy is reflected by a group of electrodes suchas NDS electrodes, charge transport is no longer unidirectional and theHACT phenomenon is corrupted or ceases altogether. Reflections occurprimarily because of mass loading of the surface and/or because ofchanges in the electrical boundary conditions (i.e., short-circuiting ofelectrical fields by metallic structures).

The latter source of reflections is in direct proportion to theelctromechanical coupling manifested by the substrate/channel/dielectriclayer (if present)/piezoelectric overlayer combination. Accordingly,increasing the electromechanical coupling by including an overlayer ofmore piezoelectric material causes the acoustic reflections present in adevice to increase.

This problem is alleviated by employing electrode configurations whichreduce or eliminate acoustic reflections through cancellation ofreflections from one electrode with reflections from a neighboringelectrode. An example of such an electrode configuration is an array ofone-eighth of an acoustic wavelength wide electrodes placed onone-fourth of an acoustic wavelength centers.

A typical HACT device channel structure suitable for use as channel 4(FIG. 1) comprises (from substrate to dielectric layer 11 and/orthin-film overlay 3 of a piezoelectric material) (i) a buffer structure,(ii) a 1000 Angstrom thick not intentionally doped (NID) Al_(x) Ga_(1-x)As layer, (iii) a 400 Angstrom thick NID GaAs layer, (iv) a 700 Angstromthick Al_(x) Ga_(1-x) As layer with an electron donor concentration (ND)of 2×10¹⁷ /cm.³ and (v) a layer of NID GaAs, typically 200 Angstromsthick. Electron transport is effected by the SAW clocking signal andoccurs within layer (iii).

The buffer structure (i, supra) may comprise layer (ii, supra), a layerof GaAs or a superlattice buffer layer comprising alternating layers ofNID GaAs and/or Al_(x) Ga_(1-x) As and/or AlAs, each in the range of20-100 Angstroms thick. The mole fraction x of Al incorporated in Al_(x)Ga_(1-x) As may range from 0% to about 50%, is typically in the rangefrom 20% to 40% and is desirably about 30%; other alloys includingelements from columns III and V of the periodic table (known as III-Vmaterials or III-V semiconductors) are also suitable as well as otheralloys such as II-VI (e.g., CdS, etc.), II-IV-VI, etc.

Typically, GaAs substrata are employed, however, the principalrequirement is that the channel layers have good lattice match to thesubstrate. Alternatively, strained or "pseudo-morphic" layers may beemployed.

Typically, homoepitaxial (growth of a material on a single crystalsubstrate of the same material such that long and short range order arepreserved in the grown layer) and heteroepitaxial (growth ofheterostructures, e.g., a first material grown on a single crystalsubstrate of a second material such that long and short range order arepreserved in the grown layer and such that these orders are related tothe long range order present in the single crystal substrate) layerssuitable for use in such structures are grown by liquid phase epitaxy orLPE, metalorganic vapor deposition or MOCVD, molecular beam epitaxy orMBE, metalorganic molecular beam epitaxy or MOMBE and/or similarprocesses as are well known in the art. Epitaxial growth services areavailable from a variety of vendors including PicoGiga of Paris, Franceand BandGap of Broomfield, Colo.

Alternatively, an epitaxial structure comprising different layers of asingle semiconductor material (i.e., homostructures) may be employed.Examples of such homostructures include thin (e.g., generally less than0.2, desirably less than 0.1 and preferably less than 0.05 of anacoustic clocking signal wavelength thick) epitaxial layers of p-, n-and/or nid-type material deposited to provide a high mobility channellayer having electrostatic boundary conditions at upper and lowersurfaces thereof which prevent mobile charge (i.e., majority carriers)from leaving the channel region during the transport process.

For example, an n-type GaAs channel layer having N_(D) =10¹⁷ /cm³ 0.2micrometers thick and bounded above and/or beneath by p-type GaAs (or,for example, Al_(x) Ga_(1-x) As) layers having N_(A) =5×10¹⁶ /cm³ 0.15micrometers thick could be employed together with appropriate biasconditions and/or bias structures and an acoustic clocking signal havinga wavelength of, for example, eight micrometers, although larger andsmaller acoustic clocking signal wavelengths could be employed as well.

Alternatively, an n-type layer comprising approximately one-halfmicrometer of n-type GaAs having N_(D) circa 5×10¹⁶ /1×10¹⁷ /cm³ may beemployed without boundary layers wherein the top surface of the channelcomprises, by way of example, a series of one-eighth of an acousticclocking signal wavelength strips of conductive material disposed alongthe acoustic clocking signal wave propagation direction and the bottomsurface is bounded by semi-insulating semiconductor material or alightly doped p-type region disposed on semi-insulating semiconductormaterial.

It is desirable that substrate 1 be semi-insulating in order to reduceinput-output coupling by capacitive coupling of the input and outputstructures to the substrate material. A broad variety of materials maybe employed as substrata, but III-V semiconductor materials arepreferred.

Important factors are (i) that the charge be transported withinapproximately one sixth and preferably one-eighth of a wavelength of theacoustic clocking signal of the top surface of the channel materials,(ii) that the charge be transported at least 200 Angstroms, desirably atleast 400 Angstroms and preferably at least 600 Angstroms below a topsurface of the semiconductor materials comprising the channel, (iii)that the material surrounding the channel be depleted of mobilecarriers, (iv) that the transported charge within the channel iselectrostatically constrained to remain in the channel, (v) that theacoustic clocking signal propagate in predominantly a single directionand (vi) that the substrate material be semi-insulating (i.e., have abulk resistivity of 10⁵ Ω-cm, or greater).

Another important factor is that the channel material and thesurrounding materials must have a bandgap E_(G) which is large enough toavoid device performance degradation by excessive generation (or "dark")current and which will not give rise to substantial impact ionizationeffects under normal device operation conditions. For example, materialshaving narrow bandgaps, such as InSb (E_(G) circa 0.16 electron Volts at300° K. and one atmosphere) are not suitable for high frequency, highperformance charge transport device channels.

Additionally, cubic crystals such as GaAs, Si, etc., provide surfacewave propagation conditions free from unusual beamsteering or lossproperties along most major crystalline axes (e.g., <100>, <110>, <111>)on most low index faces (e.g., {100}, {110}, {111}) with the exceptionof <110> propagation on {100} faces. For angles near a <110> axis, largeacoustic losses result from coupling of surface acoustic waves to alower velocity shear mode. Thus, surface acoustic wave propagation on{100} faces must be either along the <110> axis or at angles wellremoved from the <110> axes (i.e., a <100> axis).

The term "sub-surface transport" or "SST" is used herein to meanacoustic charge transport devices meeting conditions (i) and (ii) above.SST devices are distinct from MIS structures in that MIS structuresemploy confinement of (i) minority charge carriers at (ii)insulator-semiconductor interfaces by means of (iii) an externallysupplied bias.

Features (i) through (iii) are absent in SST devices, including HACTdevices, providing SST devices with advantages of decreased trapping andsubsequent re-emission (of mobile charge carriers by spatially fixedtrapping centers) and increased charge carrier mobility (chargetransport is no longer occurring at a surface of a semiconductormaterial having attendant surface states), reduced fabrication andoperation complexity and also of improved electrical input-outputisolation (i.e., freedom from capacitively coupled interference signalsat the output).

These advantages allow ACT, HACT and SST devices to provide orders ofmagnitude higher charge transfer efficiencies (e.g., 0.999+) at ordersof magnitude greater clock frequencies (e.g., 360 and 600 MHz) than arepossible for MIS-type structures (e.g., ZnO on Si, for example) withreduced spurious output signal levels. Charge carriers in SST devicesare typically either majority carriers or charge carriers confined to anNID region, similar to HACT and ACT devices.

In conventional HACT or ACT devices, the very small (k² =7.4×10⁻⁴) SAWelectromechanical coupling coefficient dictates that the bulk of the SAWenergy is mechanical energy, rather than electrical potential. As such,large amounts of acoustic power must be transduced by SAW transducer 2in order to provide voltage levels required in order to effect chargecarrier (e.g., electrons) bunching and subsequent transport of chargebunches, or packets, synchronously with the SAW, as is required formanifestation of the ACT phenomenon. The power level required to achievea given electrical potential is directly related to total acoustic powerlevel present through the electromechanical coupling coefficient, k².

As such, incorporation of more strongly piezoelectric overlayer 3,providing greater electromechanical coupling between the SAW andattendant electrical potential, results in reduced total acoustic powerrequirements to achieve a given electrical potential magnitude and soeffect the HACT phenomenon. The degree of change in acoustic powerrequirements which can be effected is illustrated by the graph given inFIG. 2, for the case of Rayleigh waves propagating in the <110>direction on a {100}-cut GaAs substrate 1, versus thickness (in acousticwavelengths) of a more strongly piezoelectric (e.g., ZnO) overlayer 3.As can be seen in FIG. 2, very thin layers 3 of ZnO result in dramaticimprovements in electromechanical coupling coefficient magnitude, toabout a thickness of appropriately 0.15 wavelengths (the couplingcoefficient varies from about 0.0078 at a thickness of about 0.1wavelength to about 0.0089 at a thickness of about 0.2 wavelengths). Forthickner ZnO overlayers 3, as can be seen in FIG. 1, the increase inelectromechanical coupling is seen to saturate at a value of about k²=0.01.

Useful thicknesses of thin-layer piezoelectric overlayers 3 (FIG. 1)range from 0.01 to 1 acoustic wavelengths, desirably range from about0.05 to about 0.3 acoustic wavelengths and preferably range from about0.07 to about 0.2 wavelengths, with thicknesses such as 0.10, 0.11,0.12, 0.13, 0.14, 0.15, 0.16 and 0.17 acoustic wavelengths beingpreferred.

More generally, the ratio of acoustic powers required to achieve thesame electrical potential in different substrates 1 and overlayers 3 isgiven by:

    P.sub.a,x /P.sub.a,y =(k.sub.x.sup.2 C.sub.sx v.sub.ox)/(k.sub.y.sup.2 C.sub.s,y v.sub.oy)

where acoustic power for material x is denoted P_(a),x, with a similarconvention applying to other substrate parameters being compared; k_(x)² refers to electromechanical coupling coefficient; C_(sx) refers tocharacteristic capacitance per finger pair per cm. and v_(ox) refers toSAW velocity for substrate 1 or a composite materials system.

For the particular case of a thin-film ZnO layer 3 in FIG. 1 on a GaAssubstrate 1, ranging from less than one to several micrometers inthickness, several different issues dictate inclusion of thin-filmdielectric layer 11, intervening between ZnO layer 3 and GaAs substrate1.

These issues devolve from poor adhesion which ZnO exhibits on GaAssubstrates 1, evidenced by observed delamination of thin-film layers 3,excellent adhesion which ZnO layers 3 exhibit on Si₃ N₄ dielectriclayers 11, excellent adhesion of Si₃ N₄ layers 11 to GaAs substrates 1,electronic effects which zinc (a rapid diffusant and a p-type dopant)and oxygen (a deep trap, removing carriers from the material andre-emitting them at random times with very long time constants) exhibitwhen incorporated into GaAs substrate 1 material and deleterious effectsof ion bombardment of GaAs substrate 1 material during sputtering (e.g.,utilized for deposition and growth of ZnO films 3), which are obviatedby inclusion of intervening layer 11 of dielectric material.

Applicants have discovered that particular advantages accrue from use ofthin film layers of hydrogenated silicon nitride and/or siliconoxy-nitride deposited by low pressure chemical vapor deposition based ondecomposition of NH₃ and SiH₄.

Specific deposition conditions are outlined in "The Elastic Propertiesof Thin-Film Silicon Nitride" by Hickernell et al., 1990 UltrasonicsSymposium Proceedings and in "The Elastic Constants of PECVD SiliconOxynitride" by Hickernell et al., 1991 Ultrasonics Symposium Proceedingswhich are incorporated herein by reference.

Specific advantages of PECVD nitride and oxynitride films include (i)very low acoustic propagation losses compared to most dielectric films,(ii) ease of patterning such films using conventional gas- orliquid-phase etching techniques, (iii) passivation of the semiconductorsurface, (iv) excellent step coverage, (v) improved semiconductorsurface breakdown performance under high electric fields and (vi) goodadhesion of the dielectric film to both the piezoelectric layer and tothe semiconductor substrate, in contrast to many dielectric films.

Acoustic propagation losses due to the inclusion of a low acousticpropagation loss film overlayers such as dielectric layer 11 aredesirably less than 4 dB/microsencond at 400 MHz, preferably less than 3dB per microsecond and optimally less than 2.5 dB/microsecond.

Inclusion of a thin film layer of a more piezoelectric material 3 onsemiconductor substrate 1 including a HACT device thus results insubstantial reduction of the acoustic energy level required in order tomanifest the HACT phenomenon. Further, some combinations of thin-filmoverlay 3 and substrate 1 materials dictate incorporation of thin-filmdielectric layer 11.

When thin-film piezoelectric layer 3 is also included over SAWtransducer 2, a reduced area on semiconductor substrate 1 is required.This occurs through well-known relationships between SAW transducerparameters and results in greater operating bandwidth for SAW transducer2. This increased bandwidth allows for greater variation in SAWfrequency. This can be used to compensate for SAW transducer frequencyshifts introduced by temperature or manufacturing variations. ReducedSAW transducer size allows for an increased number of HACT devices perwafer and so for reduced cost per device. Piezoelectric layer 3 may beapplied only in the region of SAW transducer 2 to provide increasedtransducer bandwidth.

Alternatively, dielectric layer 11 and piezoelectric overlayer 3 may becombined by, for example, providing an epitaxial layer of AlN directlyon the surface of channel 4. AlN is a wide bandgap semiconductor whichcan be prepared in insulating form and which may be grown on GaAs andsimilar materials by MOCVD, for example.

An additional advantage provided by the device architecture disclosedherein is that NDS electrodes having different element factors can bereadily provided in a single, compact structure.

This is easily provided, for example by placing electrodes 7A (FIG. 1)(i) on the surface of channel 4, where NDS elements factors such as 310(FIG. 3) obtain, placing electrodes 7B (FIG. 1) at an interface ofdielectric layer 11 and piezoelectric overlayer 3, where NDS elementfactors such as 315 (FIG. 3) obtain and placing electrodes 7C (FIG. 1)atop dielectric layer 3 to provide a response similar to 320 (FIG. 3),providing a new degree of freedom in realizing a desired frequencyresponse or family or frequency responses.

Thus, an acoustically clocked charge transport device having apiezoelectric overlayer has been described which overcomes specificproblems and accomplishes certain advantages relative to prior artmethods and mechanisms. The improvements over known technology aresignificant. The expense, complexities, and high fabrication complexityof CCDs are avoided.

Similarly, the advantages of high charge transfer efficiency, longcharge storage times, high fidelity frequency response, low clockingsignal power requirements, low dark current and additional flexibilityin selecting frequency responses for individual sense electrodes areprovided in compact form.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without departing from the generic concept,and therefore such adaptations and modifications should and are intendedto be comprehended within the meaning and range of equivalents of thedisclosed embodiments.

It is to be understood that the phraseology or terminology employedherein is for the purpose of description and not of limitation.Accordingly, the invention is intended to embrace all such alternatives,modifications, equivalents and variations as fall within the spirit andbroad scope of the appended claims.

What is claimed is:
 1. A heterostructure acoustic charge transport (HACT) device comprising:a semiconductor substrate including:a surface; a source of electric charge; acoustic wave means coupled to said semiconductor substrate, said acoustic wave means operating in response to an applied voltage source to provide an acoustic wave to transport said electric charge, said acoustic wave means including an interdigital metallic wave transducer disposed along said semiconductor substrate; a HACT channel disposed along said surface of said semiconductor substrate, said HACT channel including heteroepitaxial semiconductor layers for transporting said electric charge in response to said acoustic wave means in a particular direction; and piezoelectric means disposed over said HACT channel, said piezoelectric means facilitating transportation of said electric charge in said particular direction of said HACT channel for reducing power consumption by said HACT device, wherein said HACT device further includes:a first plurality of nondestructive sensing Schottky electrodes having a first element factor, said first plurality connected to said top layer of said plurality of heteroepitaxial layers, each of said first plurality of electrodes providing a replica of said electric charge from said source; and a second plurality of sensing electrodes having a second element factor different than said first element factor, said second plurality of sense electrodes coupled to said top layer of said plurality of heteroepitaxial layers, each of said second plurality of electrodes providing a replica of said electric charge from said source.
 2. A device as claimed in claim 1, further comprising a third plurality of sensing electrodes having a third element factor different than said first or said second element factors, said third plurality of sense electrodes coupled to said top layer of said plurality of heteroepitaxial layers, each of said third plurality of electrodes providing a replica of said electric charge from said source. 